Method for manufacturing magnetic field detection devices and devices therefrom

ABSTRACT

A method for manufacturing magnetic field detection devices comprises the operations of manufacturing a magneto-resistive element comprising regions with metallic conduction and regions with semi-conductive conduction. The method comprises the following operations: forming metallic nano-particles to obtain regions with metallic conduction; providing a semiconductor substrate; and applying metallic nano-particles to the porous semiconductor substrate to obtain a disordered mesoscopic structure. A magnetic device comprises a spin valve, which comprises a plurality of layers arranged in a stack which in turn comprises at least one free magnetic layer able to be associated to a temporary magnetisation (MT), a spacer layer and a permanent magnetic layer associated to a permanent magnetisation (MP). The spacer element is obtained by means of a mesoscopic structure of nanoparticles in a metallic matrix produced in accordance with the inventive method for manufacturing magneto-resistive elements.

This application is the US national phase of international applicationPCT/IB2004/002543 filed 30 Jul. 2004 which designated the U.S. andclaims benefit of IT TO2003A000604, dated 5 Aug. 2003, IT TO2003A000605,dated 5 Aug. 2003 and IT TO2003A000727, dated 23 Sep. 2003, the entirecontent of which is hereby incorporated by reference.

The present invention relates to a method for manufacturing magneticfield detection devices, said method comprising the operations ofmanufacturing a magneto-resistive element comprising regions withmetallic conduction and regions with semiconductive conduction.

According to the state of the art, to detect magnetic fields,magnetoresistive sensors are employed, i.e. devices whose resistance tothe passage of the electrical current varies with variations in themagnetic field whereto they are subjected. In particular, magneticsensors called AMR (Anisotropic Magneto Resistance) are known; they areusually obtained by means of a thin film of iron-nickel (permalloy),deposited onto a silicon wafer and shaped in the form of a resistivestrip.

The application of an external magnetic field determines a change in theorientation of magnetisation in the permalloy, making its magnetisationnot parallel to the current that flows in the resistive strip andthereby increasing resistance. Said AMR sensors change their resistanceby 2-3% in the presence of magnetic fields. In order effectively toappreciate the change in resistance, the AMR sensors are thus laid insuch a way as to form a Wheatstone bridge.

However, the change in resistance is linked to the occurrence of themagneto-resistive effect, present in a limited quantity of materialssimilar to permalloy.

Moreover, such sensors are not easy to integrate and miniaturise. U.S.Pat. No. 6,353,317 teaches using a porous semiconductor structure tocreate nanowires or nanotubes, which are subsequently filled withmagnetic material. FIG. 1 shows a magneto-resistive element 10,comprised in a device for detecting magnetic fields, globally designatedby the reference 15, obtained by depositing a metal into the pores of aporous semiconductor. Said magnetoresistive element 10 comprises asemiconductor substrate 11, in which are present pores 12. Inside thepores 12 are present cylinders 13 made of metallic material. To thesemiconductor substrate 11 are applied lateral electrodes 14. Thesemiconductor substrate 11 is constituted by a high mobilitysemiconductor, e.g. InAs. The operation of the device 15 is as follows.

To the lateral electrodes 14 is applied a voltage V able to determine acurrent I, which flows between the electrodes 14 and whose value isdetermined by the resistance of the magnetoresistive element 10. Saidresistance is substantially due to the current flows through themetallic cylinders 13, whose resistance is lower.

In the presence of an external magnetic field H, in the cylinders 13,due to Lorentz' force, a spatial charge distribution is achieved thatdetermines an electrical field tending to exclude the passage of currentinside them. Therefore, the value of the current I that flows in themagnetoresistive element 10 is determined by the resistance of thesemiconductor substrate 11, which is higher. Moreover, in it theelectronic paths are more tortuous and longer and this contributes tothe resistance increase in the magnetoresistive element 10. Therefore,the detecting device 15 allows to detect a magnetic field H by means ofthe sudden change, in particular the sudden increase in the resistanceof the magnetoresistive element 10 in the presence of the magnetic fieldH.

The porous semiconductor material that constitutes the substrate 11 isproduced by means of a reactive ion etching technique applied to asemiconductor wafer, whilst the metal that constitutes the cylinders 13in the pores 12 is deposited by means of an electrical depositionmethod.

However, such a procedure is quite complex and costly, involving areactive ion etching process for the creation of conducting islands inthe semiconductor.

The object of the present invention is to provide a solution enabling tomanufacture a magnetic field detection device comprising regions withmetallic conduction and regions with semiconductive conduction in simpleand economical fashion.

According to the present invention, said object is achieved thanks to amethod having the characteristics specifically recalled in the claimsthat follow.

The invention shall now be described with reference to the accompanyingdrawings, provided purely by way of non limiting example, in which:

FIG. 1 shows a schematic diagram of a magnetic field detection device;

FIGS. 2A, 2B and 2C show steps of a method for manufacturing a devicefor detecting magnetic fields according to the invention;

FIG. 3 shows a schematic diagram of a spin valve magnetic devicemanufactured according to the manufacturing method of the invention;

FIGS. 4A and 4B show, in diagram form, two different operating states ofthe device of FIG. 3;

FIG. 5 shows a schematic diagram of a spin valve magnetic deviceaccording to the invention;

FIG. 6 shows a schematic diagram of a detail of the spin valve magneticdevice of FIG. 5.

The idea constituting the basis for the method according to theinvention is to obtain the magnetoresistive element of the magneticfield detection device with a disordered mesoscopic structure ofmetallic nanoparticles in a semiconductor substrate with high mobilityand narrow band gap.

With reference to FIGS. 2A, 2B and 2C, therefore, a method is proposedfor manufacturing a magnetoresistive element 20 with similar purposesand operation to the magnetoresistive element 10, shown in FIG. 1. Saidmethod in a first step entails preparing nanoparticles or clusters ofmetallic material, through a process of synthesising metallic colloidsor another known process for synthesizing metallic nanoparticles. Saidnano-metallic particles, designated by the reference 37 in FIG. 2B,alternatively, are also available on the market and can be simplybought.

In a second step of the proposed manufacturing method, said metalnanoparticles are inserted together with an appropriate solvent in asolution 40. The solvent can be, by way of example, glycol or acetone.

A third step of the proposed method provides for rendering porous asubstrate of semiconductor material 31. In a preferred version, atemplate 38 made of anodised alumina is applied to serve as a templateon the surface of the semiconductor substrate 31. Said anodised aluminatemplate 38 is provided, by virtue of the anodisation process whereto itwas subjected, with nanometric pores 39, so it is possible to executesimultaneously spatially selective acid etchings, in particular by meansof an electrochemical etching, through the pores 39 of the anodisedaluminium template 38.

In particular, a current IA is made to pass through an acid electrolyticsolution 32 between said semiconductor substrate 31, provided with arear contact 34 which constitutes the anode, and a platinum filament 33which constitutes the cathode. In the solution, the charge can only betransported if at the electrolyte/semiconductor interface there is apassage of charge between an ion of the electrolytic solution 32,designated by the reference 35 in FIG. 2A, and positive ions 36 of thesilicon substrate 31. This takes place by means of a chemical reactionthat dissolves the anode, in the specific case the semiconductorsubstrate 31. As a consequence thereof pores 22 are developed in depthin the substrate 31 dissolving it partially.

In a preferred version of the method said acid etching is performeduntil obtaining pores 22 passing through the entire volume of thesemiconductor substrate 31.

It also possible to use other nano-porous templates instead of alumina,such as polymethylmethylacrylate (PMMA) or polymides.

A fourth step of the method, illustrated in FIG. 2 b, then provides forapplying said solution 40 containing metallic nanoparticles 37 to thesemiconductor substrate 31, now rendered porous, through a precipitationor capillary condensation process. The metallic nanoparticles 37penetrate by capillarity into the pores of the nano-porous matrix,whilst the liquid fraction of the solution evaporates, giving rise to acapillary condensation process.

Alternatively, instead of capillary precipitation or condensation, anelectrochemical plating method can be used to deposit the metallicnanoparticles 37 into the pores 22.

In a fifth step a thermal annealing process is then performed to melt oraggregate said metallic nanoparticles in a column structure or nanorod23, shown in FIG. 2C, and lower their resistance, obtaining amagnetoresistive element 20 constituted by a porous semiconductor matrixwith pores 22 filled with metallic material.

According to a further inventive aspect of the proposed method, thereplacement of the electrolytic solution 32 in the third step with thesolution 40 containing the metallic nanoparticles 37 occursprogressively without uncovering the surface of the substrate 31, i.e.leaving a sufficient layer of electrolyte 32 to cover the pores 22, andhence to prevent air or ambient gas from penetrating therein. This wouldmake it difficult for the metallic nanoparticles 37 to achieve deeppenetration.

Subsequently, in a step not shown in the figures, then, themagnetoresistive element 20 is provided with lateral contacts, similarto those shown in FIG. 1, by means of a metallic evaporation process.

The metallic nanoparticles can be made of any metal such as gold,silver, aluminium, gallium, indium, copper, chrome, tin, nickel, iron,platinum, palladium, cobalt, tungsten, molybdenum, tantalum, titanium,permalloy, as well as of other ferromagnetic alloys or other alloys withsubstantially metallic conduction.

The semiconductor substrate 31 can be laid onto any other insulatingsubstrate, e.g. silicon or glass, by the most disparate methods, such ascontinuous or pulsed electrical deposition, electrochemical methods,simple precipitation, centrifuging, thermal evaporation or electronbeam, simple or magnetron sputtering, CVD, PECVD, serigraphy.

A spin valve device which employs the manufacturing method describedabove shall now be described.

In the sector of magnetic field sensors, magnetic devices are knownwhich use the so-called ‘spin valves’. A spin valve is a devicegenerally constituted by a succession of layers of different materials.

The structure of a spin valve magnetic device 110 according to theinvention is shown schematically in FIG. 3. Said spin valve 110comprises a plurality of stacked layers of different materials. Thisplurality of layers comprises, in particular, a substrate 114, forexample a glass substrate, whereon is laid a growth layer 115, alsocalled seed layer, obtained for example with a layer of tantalum, whichacts as a seed for the growth of a permanent magnetic layer 112. Thefree magnetic layer 111 is constituted by a soft magnetic material, suchas an iron-nickel alloy, like permalloy, provided with a non permanentmagnetisation. Said free magnetic layer 111 serves the purpose oforienting its magnetisation following the external magnetic field to bemeasured. Superiorly to the free magnetic layer 111 is placed a nonferromagnetic spacer layer 113.

On the spacer layer 113 is laid a permanent magnetic layer 112. In FIG.3, said permanent magnetic layer 112 is shown comprising two layers, apinned magnetic layer 112A, also called ‘pinned layer’ and a pinningantiferromagnetic layer 116, also called ‘pinning layer’. Theantiferromagnetic layer 116 produces a short radius magnetic field thatinfluences and pins the magnetisation of the pinned layer 112A, whichcan no longer follow an external magnetic field. The set of the layers112A and 116 behaves in fact as a permanent magnet with high magneticcoercivity and provides a reference field to the spin valve 110.

The permanent magnetic field 112 can alternatively be obtained by thesimple laying of a single hard magnetic layer, for example a layer ofcobalt.

The antiferromagnetic layer 116 of the spin valve 110 is obtained, forexample, by means of a NiMn alloy. Said antiferromagnetic layer 116 isthen coated by a passivating layer 117, also made of tantalum.

The spin valve 110 shown in FIG. 3 is of the CIP (current in plane)type, i.e. to the spin valve, by means of a generator 119, is applied acurrent I that flows in planar fashion in the spacer layin the spacerlayer 113 and in the other layers of the spin valve 10. The spacer layer113 then is the layer that contributes most to determine the electricalresistance of the spin valve 10 in the absence of a magnetic field. Itis also possible to have a CPP configuration (Current Perpendicular toPlane), in which the current I is forced to traverse vertically thestacked layers of the spin valve.

The spin valve 110 shown in FIG. 3 is of the CIP (current in plane)type, i.e. to the spin valve, by means of a generator 119, is applied acurrent I that flows in planar fashion in the spacer layer 113 and inthe other layers of the spin valve 110. The spacer layer 113 then is thelayer that contributes most to determine the electrical resistance ofthe spin valve 110 in the absence of a magnetic field. It is alsopossible to have a CPP configuration (Current Perpendicular to Plane),in which the current I is forced to traverse vertically the stackedlayers of the spin valve.

In the absence of an external magnetic field, the spin valve shown inFIG. 3 is in ferromagnetic configuration, i.e. the free magnetic layer111 and the permanent magnetic layer 112 have the same direction ofmagnetisation. In the figures, the direction of the temporarymagnetisation associated with the free magnetic layer 111 is indicatedwith an arrow and the reference MT, whilst the direction of thepermanent magnetisation associated with the permanent magnetic layer 112is indicated with an arrow and the reference MP. Thus in this case thespin valve 110 has high electrical conductivity, since the path of theelectrons, designated by the reference “e” in FIG. 4 undergoessubstantially no scattering inside the spin valve device 110.

In the presence of an external magnetic field H whose direction isopposite to the reference magnetic field of the spin valve 110, as shownin FIG. 4B, which is given by the permanent magnetic field 112, the spinvalve is in anti-ferromagnetic configuration and it has low electricalconductivity. As shown in FIG. 4B, the path “e” of the electrons in thespacer layer 113 and in the spin valve is subjected to a considerablescattering phenomenon.

FIG. 5 shows a detailed diagram of the spin valve magnetic device 110according to the invention. The accessory layers, such as substrate 114and the other layers 112A, 115, 116, 117, as in FIG. 4A and 4B, are notshown for the sake of simplicity.

The spin valve 110 therefore comprises the free magnetic layer 111 andthe permanent magnetic layer 112 respectively made of a hard magneticmaterial and a soft magnetic material.

The spacer layer 123 instead is of the composite mesoscopic type, inwhich nanoparticles 124 are dispersed in a matrix structure 125.

The spin valve 120 shown in FIG. 5 is particularly suitable for a GMRsensor, so the spacer layer 123 is obtained with a composite mesoscopicstructure, in which the nanoparticles 124 made of metal, but alsopossibly of ferromagnetic and/or dielectric and/or ceramic and/orsemiconductor material, are dispersed in the matrix structure 125 with athickness ranging from a few angstrom to hundreds of nanometers. Such astructure of the spacer layer 113 allows to control the electronicscattering properties and to control the reference electrical resistanceof the device in the absence of magnetic field and of the dynamic workfield.

If a TMR spin valve has to be obtained, the spacer layer 133, shown inFIG. 6, is preferably constituted by a dielectric layer comprisinginsertions of clusters of metallic, ferromagnetic, semiconductor orother dielectric atoms. A dielectric spacer layer with mesoscopicstructure allows to control electronic tunnelling properties through themesoscopic layer of the device responsible for its resistivity and hencethe reference electrical resistance of the device in the absence ofmagnetic field and of the dynamic work field.

The spacer layer 113, both in the GMR case and in the TMR case, can beobtained by simultaneous plating of one or more elements by thermalco-evaporation, electron-beam, CVD, PECVD, sputtering and/or continuousor pulsed electrodeposition, simple precipitation, centrifuging orserigraphy.

The spacer layer 133 shown in FIG. 6, in particular, is obtained by amethod that provides for filling, by simultaneous deposition,electron-beam, CVD, PECVD, sputtering and/or continuous or pulsedelectrodeposition, simple precipitation, centrifuging or serigraphy,matrices of nano-porous materials obtained by electrochemicalself-assembly, such as anodised alumina or porous silicon.

In particular, FIG. 6 shows the spacer layer 133, which comprises amatrix of porous alumina 135, provided with pores 136, in which aredeposited by electroplating metallic nanoparticles 134 in columnstructure or nanorods.

In a preferred version, the spacer layer 133 is obtained according tothe methods of FIGS. 1, 2A, 2B and 2C.

The solution described above allows to achieve considerable advantageswith respect to prior art solutions.

A method is proposed for manufacturing magnetic field detection devices,equipped with a magnetoresistive element comprising regions withmetallic conduction and regions with semiconductive conduction, whichadvantageously provides for adopting a disordered mesoscopic structure,with separate preparation of metallic nanoparticles and subsequentapplication to a semiconductor substrate with simple and economicalprocesses.

Naturally, without altering the principle of the invention, theconstruction details and the embodiments may vary widely from what isdescribed and illustrated purely by way of example herein, withoutthereby departing from the scope of the present invention.

In a possible variant to the proposed manufacturing method, to obtain amagnetoresistive element with a disordered mesoscopic structurecomprising semiconductive areas and metallic areas having differentconduction properties according to the applied magnetic field, by virtueof the generation of a spatial charge zone in the metallic area becauseof the Lorentz force, to co-evaporate said metallic nanoparticles withinthe scope of a growth process of a semiconductor substrate. For example,it is possible to co-evaporate gold particles during the process ofgrowing a substrate of indium antimonide by Chemical Vapour Depositionor sputtering.

A device obtained with the described method can be used as a magneticfield sensor or magnetic switch, as an electromagnetic radiation sensor,as an electromagnetic radiation emitter, as a photovoltaic cell, and asa thermophotovoltaic cell.

The spin valve device described herein, moreover, advantageously allowsto control the electronic scattering properties of the deviceresponsible for its resistivity, through an appropriate selection of thetype of mesoscopic structure to be laid, both with respect to the matrixand with respect to the nanoparticles included in the matrix. A spacerlayer thus conceived allows to control and change the referenceelectrical resistance of the device in the absence of magnetic field andof the dynamic work field. Moreover, the characteristics of the spacerlayer can advantageously be studied and regulated operating on amultiplicity of composition parameters, in order simultaneously toobtain high sensitivity.

1. A magnetic device comprising a spin valve, said spin valve comprisinga plurality of layers arranged in a stack which in turn comprises atleast one free magnetic layer able to be associated to a temporarymagnetisation (MT), a spacer layer and a permanent magnetic layerassociated to a permanent magnetisation (MP), wherein said spacer layeris obtained according to a method of manufacturing a magnetoresistiveelement comprising regions having metallic conduction and regions havingsemi-conductive conduction wherein said method comprises the followingoperations: forming metallic nanoparticles to obtain said regions withmetallic conduction; providing a semiconductor substrate; chemicallyetching the semiconductor substrate to form pores in said semiconductorsubstrate; and applying said metallic nanoparticles to saidsemiconductor substrate having pores in order to obtain a disorderedmesoscopic structure, wherein said spacer layer comprises a matrix andnanoparticles, said matrix being a matrix of nano-porous materialobtained by electrochemical assembly, and wherein said matrix comprisesa porous dielectric material comprising porous silicon, and thenanoparticles are contained in pores of said porous dielectric material.2. A device as claimed in claim 1, wherein said matrix is a matrix ofdielectric material.
 3. A device as claimed in claim 1, wherein thedevice is configured to regulate its electrical properties through thecomposition of said spacer layer.
 4. A device as claimed in claim 1,wherein the device is employed in TMR applications.
 5. A device asclaimed in claim 1, wherein the substrate after being subjected tochemical etching to form pores is a nano-porous substrate.
 6. A deviceas claimed in claim 1, wherein the pores contain metallic nanoparticlesin column structure obtained by electrodeposition.
 7. A magnetic devicecomprising a spin valve, said spin valve comprising a plurality oflayers arranged in a stack which in turn comprises at least one freemagnetic layer able to be associated to a temporary magnetisation (MT),a spacer layer and a permanent magnetic layer associated to a permanentmagnetisation (MP), wherein said spacer layer is obtained according to amethod of manufacturing a magnetoresistive element comprising regionshaving metallic conduction and regions having semi-conductive conductionwherein said method comprises the following operations: forming metallicnanoparticles to obtain said regions with metallic conduction; providinga semiconductor substrate; chemically etching the semiconductorsubstrate to form pores in said semiconductor substrate; and applyingsaid metallic nanoparticles to said semiconductor substrate having poresin order to obtain a disordered mesoscopic structure, wherein saidspacer layer comprises a matrix and nanoparticles, and wherein saidmatrix comprises a porous dielectric material comprising porous silicon,and the nanoparticles are contained in pores of said porous dielectricmaterial.
 8. A device as claimed in claim 7, wherein the pores containmetallic nanoparticles in column structure obtained byelectrodeposition.
 9. A device as claimed in claim 7, wherein the deviceis configured to regulate its electrical properties through thecomposition of said spacer layer.
 10. A device as claimed in claim 7,wherein the device is employed in TMR applications.